Activation of theDrosophilaphotoresponse is a rapid process that results in plasma membrane Ca2+and Na+conductances. Ca2+functions in negative feedback regulation ofDrosophilavision including deactivation. Protein kinase C (PKC) binds directly to Ca2+and is required for deactivation. However, the consequences of disrupting phosphorylation of any individual PKC substrate in theDrosophilaretina have not been addressed. In the current work, we show that NINAC p174, which consists of a protein kinase domain joined to the head region of myosin heavy chain, is a phosphoprotein and is phosphorylatedin vitroby PKC. Mutation of either of two PKC sites in the p174 tail resulted in an unusual defect in deactivation that had not been detected previously for otherninaCalleles or other loci. After cessation of the light stimulus, there appeared to be a transient reactivation of the visual cascade. This phenotype suggests that a mechanism exists to prevent reactivation of the visual cascade and that p174 participates in this process.